The present invention relates to a method of controlling the surface potential of a photoconductive element which is installed in an electrophotographic copier and other electrostatic recording equipment to serve as an image carrier.
In an electrophotographic copier, for example, as a predetermined copying cycle repeatedly occurs, a potential is caused to remain on a photoconductive element due to the fatigue of the element even after the element has been discharged, as is well known in the art. The potential remaining on the photoconductive element, or residual potential, sequentially increases with the number of copies produced (number of copying cycles repeated). As the residual potential level reaches a certain threshold level, it causes smears to appear in the background of a copy.
One approach heretofore proposed to eliminate such smears in the background consists in measuring the residual potential on the photoconductive element by a special potential sensor, comparing the potential measured with a predetermined reference value, and correcting a developing bias voltage based on the result of comparison. This approach which relies on a potential sensor not only incurs extra costs, but also suffers from the influence of temperature and other ambient conditions. Any error in the direction of residual potential would lead to the contamination of the background of a copy.
Another approach known in the art is such that a visible pattern is formed based on a residual potential on the photoconductive element, then the density of the visible pattern is optically sensed, then residual potential on the photoconductive element is determined in terms of the density level sensed, and then at least one of a developing bias potential, a charging potential or an amount of exposure is corrected based on the residual potential level in the event of forming a document image on the photoconductive element.
The visible pattern scheme stated above has a drawback as follows. Despite that the residual potential, on a photoconductive element, usually sequentially increases on a 1,000 to 10,000 copy basis, i.e., it does not noticeably change to the negative side from a level as determined immediately before, the visible pattern mentioned above is formed by using a constant developing bias potential. This causes the amount of toner consumed to produce the visible pattern to increase with the residual potential, thereby aggravating the waste of toner. Furthermore, upon the rise of the residual potential beyond a predetermined value, the density of visible pattern becomes saturated to render accurate detection impracticable.
Another drawback with the above-described prior art scheme is that when the quantity of light issuing from an eraser is so reduced that a potential on the photoconductive element fails to be fully removed, the residual potential due to the fatigue of the element itself and the residual potential ascribable to the short quantity of light are combined together. In this condition, the visible pattern itself cannot serve as a reliable reference for the correction of potential on the photoconductive element, resulting that the amount of correction is inaccurate.
The visible pattern scheme which does not specify any color of toner for producing the visible pattern brings about another problem when applied to a multi-color electro-photographic copier in which a plurality of different colors of toner are selectively supplied. Specifically, in such an application, since it sometimes occurs that the color of toner for producing the visible pattern differs from one copying cycle to another, the reference for detection and, therefore, the level detected is changed depending upon the kind of toner used. This lowers the accuracy of correction of potential on the photoconductive element.